Ball screw drive having an axially supported threaded spindle

ABSTRACT

A ball screw ( 7, 24 ), having a threaded spindle ( 8, 28 ) which is mounted in a rolling fashion by balls ( 9, 27 ) on a threaded nut ( 10, 26 ) and which is supported on an axial bearing ( 13, 25 ), wherein the threaded spindle ( 8, 28 ) is axially supported with its first bearing surface ( 18, 23 ), which is convexly shaped with a radius of curvature (R 1 ), on a preferably conically designed second bearing surface ( 19, 22 ), wherein the first bearing surface ( 18, 23 ) and the second bearing surface ( 19, 22 ) make contact with one another along a contact path ( 37 ), wherein a quotient formed from the ratio of the radius of curvature (R 1 ) to the radius (R 2 ) of the contact path ( 37 ) assumes values of between 1.2 and 2.4 inclusive.

BACKGROUND

The present invention relates to a ball screw. Ball screws convertrotational movements into translatory movements. The present inventionalso relates in particular to an actuating device for actuating a brake,in particular parking brake for a motor vehicle.

EP 1058795 B1, for example, discloses an actuating device for a parkingbrake of a motor vehicle, in which actuating device a ball screw isprovided.

The threaded spindle, which is driven by an electric motor and which iscomprised of a plurality of spindle parts, effects a relative axialdisplacement between the threaded nut and the threaded spindle, whereinthe threaded nut, in its feed direction, exerts a pressure force on afriction pad of a disk brake. This pressure force is absorbed via anaxial bearing on the housing, on which axial bearing the threadedspindle is supported. On account of the axial pressure forces exerted,elastic deformations may occur in the housing, which elasticdeformations may cause tilting between the threaded spindle and thethreaded nut or tilting between the threaded spindle and the axialbearing. Such tilting may impair the functionality of the ball screw.

SUMMARY

It was an object of the present invention to specify a ball screw whosefunctionality is ensured even under high loads. According to theinvention, said object is achieved by means of the ball screw accordingto claim 1. Correct functionality of the ball screw is realized in thatthe threaded spindle is supported with its first bearing surface, whichis convexly shaped with a radius of curvature, on a preferably conicallydesigned second bearing surface of the axial bearing, wherein the firstbearing surface and the second bearing surface make contact with oneanother along a preferably annular contact path, and wherein furthermorea quotient formed from the ratio of the radius of curvature to theradius of the preferably annular contact path exhibits values of between1.2 and 2.4 inclusive.

A circle central point with said radius of curvature of the firstbearing surface should lie in the axial direction as close as possibleto the circle central point of the annular contact path in order topermit the easiest possible tilting. However, if—as is preferableaccording to the invention—said two circle central points lie on thespindle axis, a minimum spacing between said two circle central pointsshould be maintained; the smaller the spacing, the smaller the surfacearea available for transmitting axial forces; the larger said spacingbecomes, the larger also the surface area for the transmission of axialforces. However, this advantage is opposed by the disadvantage ofreduced tilting mobility. Furthermore, for design reasons, a minimumradius for the contact path is necessary if the threaded spindle isguided through the axial bearing; in this case, the radius of thecontact path must be greater than the radius of the spindle portionguided through.

The invention has recognized that, taking into account designconsiderations, the technical problems in conjunction with the mentionedbending moments, the ratios provided according to the invention of theradius of curvature to the radius of the contact path, eliminates thestated disadvantages.

The design according to the invention of the two bearing surfacespermits tilting between the threaded spindle and the axial bearingwithout the possibility of wear-promoting misalignment or a revolvingpoint load in the region of the axial bearing occurring. A revolvingpoint load would be encountered if no tilting were permitted in theevent of a deformation on the housing. A bending moment introduced intothe axial bearing would then duly act statically; on account of therotating spindle with the likewise rotating axial bearing, the locationof the introduced bending moment would pass with each revolution, suchthat a revolving point load in relation to the axial bearing would berealized. The invention avoids this disadvantage: this is because, onaccount of the design according to the invention of the first and secondbearing surfaces of the axial bearing, tilting between the first andsecond bearing surfaces is permitted, such that during a rotation, awobbling movement takes place between the first and second bearingsurfaces, without undesired bending moments being introduced into theaxial bearing.

With the preferred conical design of the second bearing surface, anannular contact path between the two bearing surfaces is obtainedwithout problems, wherein with a slight tilting of the threaded spindlerelative to the housing, the contact path can easily drift and deviatefrom its ideal circular path. The invention prevents the transmission ofbending moments into the axial bearing in the event of tilting. Saidcontact path is also the line of greatest contact pressure which can bedetermined by the method of Hertzian stress. Without tilting, saidcontact path has a purely circular shape.

In refinements according to the invention, the threaded spindle may beof single-piece or else multi-piece design, wherein the threaded spindlemay have a radially tapered spindle portion which may be designed forbeing driven by a motor. A shoulder may be formed at the transition fromthe radially enlarged spindle portion which interacts with the threadednut to the tapered spindle portion, on which shoulder the first convexbearing surface is formed.

As viewed in longitudinal section through the threaded spindle, thefirst bearing surface is convexly arched with a radius of curvature.Said convex arching is accordingly evident, in the case of ball screwsaccording to the invention, in longitudinal section through the ballscrew. The circle central point preferably lies with the radius ofcurvature of the first bearing surface on the spindle axis of thethreaded spindle. Likewise situated on said spindle axis is the circlecentral point of the annular contact path. The two central points arehowever axially spaced apart from one another. The contact path and itscentral point lie on a plane arranged perpendicular to the spindle axis.

According to the invention, the quotient formed from the ratio of theradius of curvature of the first bearing surface to the diameter of thecontact path is specified between the values of 1.2 and 2.4 inclusive.Within said values, the mentioned tilting movements can be permittedwithout impairment of the functionality of the ball screw. The valuesaccording to the invention relate to the load-free starting situation inwhich contact is duly produced between the first and second bearingsurfaces along the contact path, but there is no tilting and nosignificant axial loading.

Both bearing surfaces rotate with the driven threaded spindle.

It has been found that ball screws according to the invention areparticularly suitable as actuating devices in a brake of a motorvehicle, in particular in so-called immobilizing brakes or parkingbrakes. In such arrangements, as a result of high pressure forces,elastic deformations may arise in the region of the brake caliper, as aresult of which tilting between the threaded spindle and the threadednut, or tilting between the threaded spindle and the axial bearingsupported on the housing, may occur. Said tilting angle may beapproximately 0.5°. It has been found that, for such a tilting angle,the first convex bearing surface and the second, preferably conicalbearing surface are designed such that the quotient formed from theratio of the radius of curvature to the radius of the contact pathassumes values of between 1.4 and 1.6 inclusive. Here, tilting occurswithout an introduction of bending moments into the axial bearing. Theradius of the contact path is approximately between 7 mm and 10 mm.

If ball screws according to the invention are used in the parking brakementioned further above, considerable pressure forces can be transmittedin the axial bearing when the brake is activated. In order that tiltingas a result of elastic deformation on the housing is permitted evenunder high axial pressure forces, one refinement according to theinvention provides that at least one of the two first and second bearingsurfaces is provided with depressions for receiving lubricant. In suchrefinements according to the invention, there is accordingly alubricating film along the contact path, such that tilting is possible.

The axial bearing may have a support disk which is connected to thethreaded spindle in a positively locking manner in the rotationaldirections, the second bearing surface being formed on that side of saidsupport disk which faces toward the threaded nut. Said support disk ispreferably arranged, so as to be capable of performing a wobblingmotion, on the threaded spindle in order to permit the described tiltingbetween the axial bearing and the threaded spindle, without undesiredmoments being introduced into the axial bearing. Said support disk maythen be provided, on the side facing away from the threaded nut, with anaxial bearing surface which is formed either directly on the supportdisk or else on a bearing disk which adjoins the support disk. In thisway, the threaded spindle is rotatable relative to a housing but issupported on said housing in the axial direction.

It has accordingly proven to be a problem in brake devices that bendingof the brake caliper occurs as a result of the considerable actuatingforces when the brake pads are pressed against the brake disk. Saidbrake caliper bends “open”, that is to say, in effect, the ball screwwhich is fixedly mounted on the brake caliper is supported against thebrake disk and, at a sufficiently high axial contact pressure, bends thebrake caliper open. This results in an axial offset, together with angleerrors, of the nut with respect to spindle and any axial and radialbearings. This leads to a highly uneven load distribution in thebearings by means of which the ball screw is rotatably mounted on thebrake caliper, which can lead to premature failure.

Refinements according to the invention in actuating devices for parkingbrakes may provide that the threaded nut is connected to a piston whichhas the brake pad, which piston is mounted on the threaded nut so as tobe tiltable relative to the threaded nut, and/or that the threadedspindle is mounted on the brake caliper so as to be tiltable relative tothe brake caliper.

In said parking brake, there is a certain degree of relative mobility ofdifferent components which are loaded as a result of the operation ofthe ball screw during braking, which mobility permits a change in therelative position of the components with respect to one another, wherebyany axial offset or a bearing load can be at least partiallycompensated. It is thus provided, according to one alternative of theinvention, that the threaded nut is connected to a piston or said pistonis mounted directly on the threaded nut, not rigidly but such that saidpiston can be tilted about the spindle longitudinal axis (whichcoincides with the nut longitudinal axis). That is to say, the pistoncan tilt or pivot slightly relative to the threaded nut in everydirection. As a result, during the spreading of the brake caliper, thespindle axis and therefore also the nut axis is duly likewise tiltedslightly, but said tilting does not directly also act on the coupledpiston. In fact, said piston can tilt slightly relative to the nut, as aresult of which load compensation is obtained in said region. A movablesystem is thus realized here.

Said tiltability of the threaded nut may be provided in addition to thedescribed tilting mobility according to the invention in the axialbearing.

If the brake caliper is now spread apart, the caliper bending can becompensated as a result of an at least partial compensation of theposition change by means of a slight tilting of the spindle in themovable mounting on the brake caliper. That is to say that, in saidregion too, a compensating movement facility is realized which leads toa relief of load from the bearing.

In a refinement of the invention, in the case of tiltable mounting ofthe piston on the nut, it may be provided that, to realize said tiltablemounting, a first conical guide surface is provided on the nut and asecond conical guide surface is provided on the piston, which guidesurfaces bear against one another, wherein at least one of the guidesurfaces is crowned. The nut and the piston make contact with oneanother only in the region of the guide surfaces which are bothbasically conical, such that a centering action is obtained. At leastone of the guide surfaces, however, has a slight crowning, which makesit possible for the piston to tilt slightly (for example by 0.5° or lessduring operation) relative to the nut. It is also conceivable for bothguide surfaces to be crowned. The piston itself is expediently of hollowcylindrical design, with the threaded spindle being held in the pistonpreferably over the entire length of its threaded section. This leads toa substantially closed system, and the nut is consequently encapsulated,in effect, in the piston.

BRIEF DESCRIPTION OF THE DRAWINGS

Two exemplary embodiments of the invention are illustrated in thedrawing and will be described in more detail below. In the drawing:

FIG. 1 shows a diagrammatic, sectional illustration of a brake devicehaving a ball screw according to the invention in the unloaded state,

FIG. 2 shows an enlarged detail view of the region II from FIG. 1,

FIG. 3 shows an enlarged detail view of the region III from FIG. 1, and

FIG. 4 shows the brake device from FIG. 1 in the loaded state withelements tilted relative to one another,

FIG. 5 shows, in section, a further brake device having a ball screwaccording to the invention,

FIG. 6 shows the ball screw from FIG. 5, and

FIG. 7 shows an enlarged detail from FIG. 6,

FIG. 8 shows individual parts of the ball screw from FIG. 6,

FIG. 9 shows a further individual part of the ball screw from FIG. 6,

FIG. 10 shows the ball screw according to the invention in a partiallycut-away illustration, and

FIG. 11 shows the ball screw according to the invention from FIG. 10 incross section along the section line XI-XI.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a brake device 1 according to the invention of the typewhich may be implemented as a parking brake or immobilizing brake in amotor vehicle, for example. The brake device 1 comprises a brake disk 2,which is connected in a known way to the wheel, and a brake caliper 3 ofsubstantially C-shaped cross section, which fits over the brake disk 2.Accommodated in said brake caliper are two brake pads 4, 5, which arepositioned on both sides of the brake disk 2 arranged between them and,for the purpose of braking, bear firmly against the latter, clamping thebrake disk between them. FIG. 1 shows the release position, that is tosay when the brake disk 2 is not clamped and the brake disk 2 can rotatefreely between the two brake pads 4, 5, even though these are restingdirectly against the brake disk for the sake of the illustration. Inactual fact, there is a minimal gap between the brake disk 2 and thebrake pads 4, 5, allowing free rotation in the release position.

FIG. 1 furthermore shows a ball screw 7 according to the invention,which is accommodated in a portion 6 of the brake caliper 3 that may beformed in the manner of a housing and which comprises a threaded spindle8, on which a threaded nut 10 runs in a manner guided by balls 9, theballs 9 circulating continuously and being constantly returned by meansof at least one ball return element 11. The spindle 8 is connected to adrive motor (not shown in any more detail here), which is preferablyarranged in the region of the outside of the housing-like portion 6 andthe output shaft of which is at an angle of 90°, for example, to thethreaded spindle 8. The output shaft of said drive motor is coupled tothe threaded spindle 8 by way of a cardan joint, which allows thethreaded spindle 8 to be motor-driven. The threaded spindle 8 isfurthermore rotatably mounted in a fixed position on the brake caliper 3by means of a radial bearing 12 and an axial bearing 13, in the presentcase in the form of a needle-roller bearing.

The threaded nut 10, for its part, is coupled to a piston 14, and thesaid piston rests on the front end edge of the threaded nut 10, that isto say is supported there. The movable brake pad 5 is arranged on thepiston 14. If the drive motor (not shown in any more detail) is nowactivated, by actuation of a suitable actuating element on the vehicle,in order to actuate the brake device and hence to fix the brake disk 2,the threaded spindle 8 is moved by the drive motor and rotates, with theresult that the threaded nut 10 travels along the threaded spindle 8,being guided by the balls 9 in the process, that is to say moves to theleft in FIG. 1. During this process, the piston 14 seated on the endface of the threaded nut 10, and together with it the brake pad 5, ispushed to the left, with the result that it is brought firmly intocontact with the brake disk, which is supported against the other brakepad 4, whereby the said brake disk is fixed between the two brake pads4, 5.

FIG. 2 shows on an enlarged scale a detail view of the seating region ofthe piston 14 on the threaded nut 10. The piston 14 has a conical guidesurface 15, opposite which is a second guide surface 16 on the end ofthe threaded nut 10, the said second guide surface likewise beingconical in terms of its basic shape but having a crowned or convexexternal form. This means that there is no extensive contact here butonly linear bearing of guide surface 15 on guide surface 16. The effectis that the piston 14 is seated in a movable fashion on the nut 10, thatis to say guide surface 15 can move on guide surface 16 owing to thelinear support. The piston 14 can therefore tilt relative to thethreaded nut 10 and a movable bearing arrangement is achieved, withlubrication by means of a suitable lubricant to reduce friction.

As FIG. 3 shows in an enlarged detail view, a bearing arrangement whichis likewise movable is achieved in the region of support of the threadedspindle 8 on the brake caliper 3. As explained, the threaded spindle 8is supported on the wall 17 of the brake caliper, on the one handradially by means of the radial bearing 12 and, on the other hand, bymeans of the axial bearing 13. This axial bearing comprises a firstbearing disk 18 (housing disk), which is arranged in a fixed position onthe wall 17, and a second bearing disk 19 (shaft disk), which runs onthe first bearing disk 18 by way of needle rolling bodies 20. Bearingdisk 19 has an axial projection 21, which has a conical second bearingsurface 22 that, like guide surface 16 in the arrangement for supportingthe piston 14 on the threaded nut 10, has a crowned convex surface witha basic shape that is preferably substantially conical.

The threaded spindle 8, for its part, has a first, convex bearingsurface 23. It is therefore evident in this case also that a movablebearing arrangement is achieved since, here too, the first bearingsurface 23 rests on the second bearing surface 22 only along a line butnot over an area. The effect is that the threaded spindle 8 can tiltslightly relative to the positionally fixed axial bearing 13,specifically relative to the positionally fixed bearing disk 19,lubrication likewise being provided. This tilting is made possible bythe fact that the threaded spindle 8 is likewise accommodated with acertain play in the radial bearing 12, or the radial bearing, a plasticplain bearing for example, allows a certain tilting. During operation,when the caliper expands owing to the forces that are acting, the tiltangle is in a range of significantly <0.5° per movable bearing locationand, as a result, the plain bearing 12 is not subjected to significantloads.

Of course, it is possible with both bearing locations to implement thecrowning on the respective other guide surface or to make both guidesurfaces crowned.

Thus, in the brake device 1 according to the invention, two movablebearing locations are implemented, namely in the region of the seatingof the piston 14 on the nut 10 on the one hand, and in the region of theseating of the threaded spindle 8 on the axial bearing 13 on the otherhand. The effect is then that tilting of the relevant axes, which ispresent in known brake devices and results in high bearing loads thatcan lead to premature bearing failure, can be compensated to a largeextent, thus making it possible to significantly reduce bearing loads.

In the unloaded position shown in FIG. 1, the three longitudinal axes ofthe threaded spindle 8, the brake caliper 3 or, more specifically, thepreferably cylindrical housing-like portion 6, and the piston 14coincide and are denoted in this Figure as a common axis with the letterA.

If the motor (not shown) is now used to activate the threaded spindle 8and, by means of the latter, the piston 14 and with it the brake pad 5is pressed against the brake disk 2, the brake caliper 3 is expanded orspread apart to a greater or lesser extent, depending on the contactforce, as shown in FIG. 4. As can be seen, the brake caliper 3 expandsand, on the one hand, a slight gap 24 is formed in the region of brakecaliper contact with the first brake pad 4, and, as can also clearly beseen, portion 6 of the brake caliper 3 adopts an angled positionrelative to the piston 14. At this point, it should be pointed out thatFIG. 4 shows a significantly exaggerated expansion and tilting ofcomponents compared with that which occurs in reality, this being forthe sake of illustration.

By virtue of the two separate instances of mobility or movable bearingarrangements that are implemented, however, this severe angular offsetcan be effectively split up and the load acting on the axial bearing canbe significantly reduced. This is because, on the one hand, the tiltingof the brake caliper 3, that is to say its spreading apart, has theeffect that the piston 14 tilts slightly relative to the nut 10, thisbeing obtained by means of the movable seating of the piston 14 on thenut 10 via the guide surfaces 15, 16, as shown in detail in FIG. 2. Inthe same way, there is slight tilting of the seating of the threadedspindle 8 on the axial bearing 13 or bearing disk 19 by virtue of themovable bearing arrangement implemented there, as shown in FIG. 3. Heretoo, there is therefore an albeit slight relative movement or tiltingmovement. That is to say that the piston 14, the threaded nut 10, thethreaded spindle 8, and the axial bearing 13 or bearing disk 19consequently adjust relative to one another in pairs under the effect ofload and consequently there is splitting and hence, at the same time, alocal reduction of the individual tilt angles. The movement of the axialbearing 13 relative to the threaded spindle 8 also has the effect thatthe threaded spindle 8 moves or tilts relative to the radial bearing 12,as is likewise illustrated in FIG. 4. While all the longitudinal axescoincide in FIG. 1 as described, there is now an axial offset owing tothe expansion of the brake caliper, but this is significantly less owingto the instances of mobility achieved than it would be with a rigidbearing arrangement. As can be seen, the individual axes A₁ of the brakecaliper 3, A₂ of the ball screw 7 or threaded spindle 8, and A₃ of thepiston 14 no longer coincide, but the respective axial offset isnevertheless relatively small. The maximum skewing or tilting of about0.5° of the brake caliper axis relative to the normal to the brake diskwhich occurs in actual operating conditions can be well compensated bythe decoupling of the elements which is provided for by the invention,that is to say by their mobility relative to one another, with theresult that, overall, either the ball screw can be constructed withsomewhat smaller dimensions and/or the service life of the bearingsincreases significantly.

FIGS. 5 to 11 show a further brake device having a ball screw 24according to the invention. In this arrangement, the invention may alsobe referred to as an actuating device for a parking brake.

Where components illustrated here correspond to those of the exemplaryembodiment described above, the same reference numerals are used.

FIG. 5 shows, in section, a parking brake or immobilizing brake havingthe ball screw 24 according to the invention. Here, an axial bearing 25is provided which is modified in relation to the preceding exemplaryembodiment.

The ball screw 24 according to the invention with the axial bearing 25is shown clearly in section in FIG. 6. A threaded nut 26 is mounted in arolling fashion on a threaded spindle 28 in a known way by means ofballs 27. The threaded spindle 28 has, outside its portion whichinteracts with the threaded nut 26, a radially stepped spindle portion29 which is provided, on the axial end thereof, with a polygon 30. Agearing (not shown here) may be connected at the drive output side tosaid polygon 30.

FIG. 6 also shows that the threaded spindle 28 is guided with itsspindle portion 29 through the axial bearing 25. The axial bearing 25comprises a support disk 33 and an axial rolling bearing 38 in whichrollers 39 are arranged between two bearing disks 40, 41. One bearingdisk 40 bears against the support disk 33, and the other bearing disk 41is supported against the housing-side portion 6.

FIG. 7 shows an enlarged detail of the ball screw 24 and of the axialbearing 25. The threaded spindle 28 is provided with a shoulder 31 atthe transition to the radially recessed spindle portion 29. Saidshoulder 31 has a bearing surface 32 which is convexly shaped with aradius of curvature. A support disk 33 of the axial bearing 25 isarranged on the threaded spindle 28 for conjoint rotation therewith, butsuch that it can perform a wobbling motion, via a toothing 34. Thesupport disk 33 is provided, on its end side facing toward the firstbearing surface 32, with a conical opening 35 which forms a secondbearing surface 36.

The spindle axis S is indicated in FIG. 7. The radius of curvature R1 ofthe first bearing surface 32 intersects the spindle axis S. The twobearing surfaces 32, 36 make contact with one another along an annularcontact path 37, the central point of which likewise lies on the spindleaxis S. Said annular contact path 37 has a radius R2. It can be seenfrom FIG. 7 that the two radii R1 and R2 are arranged spaced apart fromone another on the spindle axis S. The radius R1 is larger than theradius R2, wherein according to the invention, a quotient formed fromthe ratio of the radius R1 to the radius R2 assumes values between 1.4and 1.6 inclusive. A circle drawn with the radius of curvature R1 liesin the plane of the page. A circle drawn with the radius of curvature R2lies in a plane arranged perpendicular to the plane of the page.

FIG. 8 shows the situation in which, owing to an elastic deformation ofthe brake caliper 3 or of the housing-like portion 6, the support disk33 is tilted relative to the threaded spindle in 28 by approximately0.5°, wherein in the illustration, said tilt is illustrated on anexaggerated scale. Undesired loading of the axial bearing 25 with abending moment is accordingly prevented. The support disk 33 isaccordingly arranged on the threaded spindle 28 so as to be capable ofperforming a wobbling motion; said support disk can tilt about axesperpendicular to the spindle axis, and can transfer torques for thetransmission of torques between support disk 33 and threaded spindle 28.

FIGS. 9 a, 9 b, 9 c show the support disk 33 in two views and inlongitudinal section. In FIG. 9 b, pockets 42 for receiving lubricantare provided in the wall of the conical opening 35. A lubricating filmis thus built up in the contact path 37, which lubricating film promotesfree-moving tilting of the two bearing surfaces 32, 36.

FIG. 10 shows the ball screw according to the invention, with threadednut 26 and support disk 33 illustrated in partially cut-away form. Here,it is possible to see a circumferential stop 43 for the threaded nut 26,which stop will be described in more detail below.

It can be seen from FIG. 10 that the support disk 33 is provided, on itsend side facing toward the threaded nut 26, with an axial projection 44.Said axial projection 44 engages into a recess 45 of the threaded nut26.

FIG. 11 clearly shows the recess 45, which extends in thecircumferential direction over a relatively large circumferentialsegment. In one circumferential direction, the recess 45 is delimited bya tooth 46 which is integrally formed on the threaded nut 26 and whichis directed radially inward. It can also be seen from FIG. 11 that theprojection 44 is arranged in a stop position in which it abuts against afirst stop surface 47 of the tooth 46.

In the axial direction, the recess 45 is delimited by a base 54 formedin one piece with the threaded nut 26. The recess is delimited in theradial direction by a circumferential wall 55 formed in one piece withthe threaded nut 26.

Said stop 43 prevents the threaded nut 26 from being able to be clampedaxially to the support disk 33. This is because, before end surfaces,which face toward one another, of the threaded nut 26 and of the supportdisk 33 come into contact with one another, the projection 44 abutsagainst the first stop surface 47 of the tooth 46.

The recess 45 extends over a circumferential angle of greater than 180°,such that the projection 44, upon a screw-type relative rotation withrespect to the threaded nut 26, protrudes into said recess 45.

The circumferential stop 43 is designed such that, in the stopsituation, a minimum spacing a is maintained between the threaded nut 26and the support disk 33, such that at any rate axial clamping betweenthe threaded nut 26 and threaded spindle 28 is prevented. FIG. 10denotes the minimum spacing a which is provided between the two endsurfaces, which face toward one another, of the threaded nut 26 and ofthe spindle disk 33.

In particular, it can be seen from FIG. 10 that the projection 44 andthe first stop surface 47 overlap one another in the axial direction.Said axial overlap is on the one hand smaller than the overall axialextent of the axial projection 44, such that in any case, theabovementioned minimum spacing a is ensured. On the other hand, saidaxial overlap is larger than the axial extent of the projection 44 minusthe axial minimum spacing a between the stop 43 and the threaded nut 26.Furthermore, the axial extent of the projection 44 is at most as largeas the pitch of the ball screw in order to keep the bending momentsacting on the projection 44 low at the instant of abutment against thefirst stop surface 47.

To prevent radial forces being generated owing to the abutment in thestop situation, in the stop position, a second stop surface 48 formed onthe projection 44 and the associated first stop surface 47 of the tooth46 are arranged in a common plane which contains the spindle axis.

The recess 45, which in the exemplary embodiment is formed on the endside of the threaded nut 26, extends in the circumferential directionover an angle formed from a quotient of the ratio of the abovementionedaxial overlap to the pitch of the threaded spindle, multiplied by 360°,wherein to determine the angle, the axial overlap and the pitch of thethreaded spindle are both designated using the same unit of length.

It can also be seen from FIG. 10 that in each case one optical marking49, 50 is formed on the threaded nut 26 and on the support disk 33.Here, said markings 49, 50 are small depressions formed on the outercircumference. Said markings 49, 50 permit simple assembly of the ballscrew 24, as will be explained in more detail below.

For correct functioning of the stop 43, the rotational position of thesupport disk 33 with respect to the threaded spindle 28 is ofsignificance. For example, if, in the exemplary embodiment, the supportdisk 33 were arranged rotated counterclockwise about the threadedspindle by 90°, a situation could arise in which the threaded nut 26 andthe support disk 33 abut against one another at the end side before thestop 43 has taken effect in the circumferential direction. Accordingly,correct rotational positioning of a stop part 51 with respect to thethreaded spindle 28 is of significance. In the exemplary embodiment, thestop part 51 is formed by the support disk 33.

It can be seen from FIG. 11 that the toothing 34, already mentionedfurther above, between the support disk 33 and the spindle portion 29 ofthe threaded spindle 28 is provided for transmitting torques. Saidtoothing 34 allows the support disk 33 to be placed onto the spindleportion 29 in a plurality of rotational positions. Said toothing 34 isformed here by an external toothing 52 on the outer circumference of thespindle portion 29 and by an internal toothing 53 on the innercircumference of the support disk 33.

A tooth flank angle α of the external toothing 52 or of the internaltoothing 53 is designed to be as small as possible, such that thesteepest possible tooth flanks are formed. Steep tooth flanks facilitatethe tilting mobility, described further above, of the support disk 33with respect to the threaded spindle 28. The finer the toothing, themore rotational positions can be set.

For assembly of the ball screw 24, the threaded nut 26 may firstly bescrewed onto the threaded spindle 28 until the threaded nut 26 hasreached its intended stop position. The support disk 33 may then beplaced onto the spindle portion 29 and rotated relative to the threadedspindle 28 and the threaded nut 26 until the two markings 49, 50 arearranged in alignment with one another. The support disk 33 may then bepushed axially further in the direction of the threaded nut 26, whereinthe internal toothing 53 engages into the external toothing 52. It isalso conceivable for two markings to be provided for example on thesupport disk 33, between which the marking 49 of the threaded nut 26should be arranged. In this way, an angle is defined within which anadmissible rotational position for the support disk 33 relative to thethreaded spindle 28 is provided.

The assembly depicted here may take place in an automated fashion,wherein the markings 49, 50 can be detected by means of suitablemeasurement sensors. When said markings 49, 50 are in alignment with oneanother, by means of suitable control, the next assembly step can betriggered and the support disk 33 can be pushed with its internaltoothing 53 onto the external toothing 52 of the spindle portion 29.

The ball screw may be formed without a ball return facility. This meansthat the balls are arranged in a non-endless ball channel and can merelyroll back and forth between the ends of said ball channel. In theexemplary embodiment, a helical compression spring may be inserted intothe ball channel, one end of which spring is supported against the tooth46 and the other end of which spring is loaded against the final ball.During load-free ball screw operation, all the balls can bespring-loaded in the direction of the end of the ball channel under theaction of a spring force of the helical compression spring.Alternatively, a ball screw may also be used which has, as is known, aball return facility: the balls circulate in a continuous manner inendless ball channels. The ball channel is formed from a load portion,in which the balls roll under load on ball grooves of the threaded nutand of the threaded spindle, and a return portion, in which the ballsare returned from an end to a beginning of the load portion. The returnportion may be formed, in a known way, by a diverting pipe on the outercircumference of the threaded nut, or else by diverting pieces which areinserted in the wall of the threaded nut. Said diverting pieces connectan end of a common winding of the load portion to the beginning thereof.

In the exemplary embodiment, the threaded nut 26 with the recess 45 andthe tooth 46 is formed from a case-hardened steel in the semi-hot state.Semi-hot forming is carried out in a temperature range from 750° C. to950° C. For semi-hot forming, prefabricated untreated parts may beinductively heated and formed on partially multi-stage presses.

Here, the ball groove is produced in a cutting process by turning.Alternatively or in addition, the ball groove may also be produced bythread rolling. The finished threaded nut is subsequently case-hardened.

The support disk 33 is likewise produced in a non-cutting process, inparticular in the semi-hot forming process. It can be seen in particularfrom FIG. 9 that the axial projection is approximately half pushedthrough. This means that material of the support disk 33 is formed outof the disk-shaped part, wherein the support disk 33 is provided, on itsend side facing away from the projection, with a cavity.

LIST OF REFERENCE SYMBOLS

1 Brake device

2 Brake disk

3 Brake caliper

4 Brake pad

5 Brake pad

6 Housing-like portion

7 Ball screw

8 Threaded spindle

9 Balls

10 Threaded nut

11 Ball return element

12 Radial bearing

13 Axial bearing

14 Piston

15 Conical guide surface

16 Guide surface

17 Wall

18 First bearing disk

19 Second bearing disk

20 Needle rolling bodies

21 Axial projection

22 Second bearing surface

23 First bearing surface

24 Ball screw

25 Axial bearing

26 Threaded nut

27 Ball

28 Threaded spindle

29 Spindle portion

30 Polygon

31 Shoulder

32 First bearing surface

33 Support disk

34 Toothing

35 Conical opening

36 Second bearing surface

37 Contact path

38 Axial rolling bearing

39 Roller

40 Bearing disk

41 Bearing disk

42 Pocket

43 Stop

44 Projection

45 Recess

46 Tooth

47 First stop surface

48 Second stop surface

49 Marking

50 Marking

51 Stop part

52 External toothing

53 Internal toothing

54 Base

55 Circumferential wall

A Common axis

A₁ Axis of the brake caliper

A₂ Axis of the ball screw

A₃ Axis of the piston

R1 Radius of curvature of the first bearing surface

R2 Radius of the contact path

S Spindle axis

The invention claimed is:
 1. A ball screw comprising a threaded spindlewhich is mounted in a rolling fashion by balls on a threaded nut andwhich is supported on an axial bearing, the threaded spindle is axiallysupported with a first bearing surface, which is convexly shaped with aradius of curvature (R1), on a conically designed second bearingsurface, wherein the first bearing surface and the second bearingsurface make contact with one another along a contact path, and aquotient formed from a ratio of the radius of curvature (R1) to theradius (R2) of the contact path has values of between 1.2 and 2.4inclusive.
 2. The ball screw as claimed in claim 1, wherein the quotientis between 1.4 and 1.6 inclusive.
 3. The ball screw as claimed in claim1, wherein a central point of the radius of curvature (R1) lies on alongitudinal axis (S) of the threaded spindle.
 4. The ball screw asclaimed in claim 1, wherein at least one of the first and second bearingsurfaces is provided with pockets for receiving lubricant.
 5. The ballscrew as claimed in claim 1, wherein the axial bearing has a supportdisk which is connected to the threaded spindle in a positively lockingmanner in a rotational direction, the second bearing surface beingformed on a side of said support disk which faces toward the threadednut.
 6. The ball screw as claimed in claim 5, wherein the support diskis arranged to perform a wobbling motion on the threaded spindle.
 7. Theball screw as claimed in claim 5, wherein the support disk is provided,on a side facing away from the threaded nut, with an axial bearingsurface which is formed directly on the support disk or on a bearingdisk which adjoins the support disk.